Method and apparatus for dry-etching half-tone phase-shift films, half-tone phase-shift photomasks and method for the preparation thereof, and semiconductor circuits and method for the fabrication thereof

ABSTRACT

A dry-etching method comprises the step of dry-etching a metal thin film as a chromium-containing half-tone phase-shift film, wherein the method is characterized by using, as an etching gas, a mixed gas including (a) a reactive ion etching gas, which contains an oxygen-containing gas and a halogen-containing gas, and (b) a reducing gas added to the gas component (a), in the process for dry-etching the metal thin film. The dry-etching method permits the production of a half-tone phase-shift photomask by forming patterns to be transferred to a wafer on a photomask blank for a chromium-containing half-tone phase-shift mask. The photomask can in turn be used for manufacturing semiconductor circuits. The method permits the decrease of the dimensional difference due to the coexistence of coarse and dense patterns in a plane and the production of a high precision pattern-etched product.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for dry-etching a metal thinfilm or a chromium-containing half-tone phase-shift film and morespecifically to a method and an apparatus for dry-etching such a metalthin film, which is applied to a process for preparing achromium-containing half-tone phase-shift photomask which is used infabricating, for instance, a semiconductor device as well as a methodfor dry-etching a chromium-containing half-tone phase-shift film whichis used in a pattern-etching process for forming a fine pattern of ametal thin film such as fine electrode patterns (for a flat paneldisplay (FPD) or the like) and color filters. In addition, the presentinvention also pertains to a chromium-containing half-tone phase-shiftphotomask which is provided with a pattern formed using such adry-etching method and a method for preparing the photomask as well as asemiconductor circuit fabricated using such a photomask and a method forfabricating the semiconductor circuit.

2. Description of the Prior Art

As a photomask blank for chromium-containing half-tone phase-shiftphotomasks, there have been known, for instance, those having such astructure as shown in FIG. 1, which comprises a glass substrate a formedfrom, for instance, synthetic quartz glass; a light-shielding film or ahalf-tone phase-shift film b consisting of a thin film of a metal suchas chromium or chromium oxide, formed on the surface of the substrate;and a resist layer c of a light-sensitive/electron-sensitive resin,which is formed on the light-shielding or half-tone phase-shift film.The glass substrate a may serve as a support for patterns and therefore,must have a variety of desired characteristic properties such as hightransmittance, high uniformity, defect-free characteristics, resistanceto washing and excellent flatness. In addition, the film b may serve asa light-shielding material for patterning or a phase-shift film andtherefore, should satisfy the desired requirements for variousproperties such as etching controllability, uniformity, defect-freecharacteristics, resistance to washing, low stress and high adhesion tothe glass substrate. Moreover, the resist layer c has a role as a filmfor forming a light-shielding film or a half-tone phase-shift film andaccordingly, should have a variety of desired characteristic propertiessuch as high-sensitivity/ high resolution, resistance to etching,uniformity, defect-free characteristics and high adhesion to thelight-shielding film or the film for forming a half-tone phase-shiftfilm.

A photomask provided thereon with a fine electric circuit pattern hasbeen prepared by wet-etching or dry-etching a chromium light-shieldingor half-tone phase-shift film using a photomask blank having such astructure according to the electron beam patterning process or the laserbeam patterning process. An example of such a mask-processing scheme isshown in FIG. 2. In this respect, the mask-processing scheme shown inFIG. 2 relates to a light-shielding film of chromium, but the processingscheme for the half-tone phase-shift mask is almost identical to thatdepicted on FIG. 2 and further details of the processing scheme forforming a half-tone phase-shift mask are described in JapaneseUn-Examined Patent Publication (hereinafter referred to as “J.P. KOKAI”)No. Hei 7-140635, the disclosure of which is hereby incorporated byreference.

In the wet-etching, there have recently been highlighted a limit in thedimension control due to the undercut and a limit in the verticality ofthe etched cross section, and the dry-etching technique has thus beenwidely used instead.

The dry-etching methods for preparing a photomask and the dry-etchingapparatus for practicing the methods are described in, for instance,J.P. KOKAI No. Hei 6-347996, the disclosure of which is herebyincorporated by reference. In this dry-etching technique, a chromiumfilm is etched using a gas comprising, for instance, chlorine gas towhich oxygen gas is added, as a reactive ion etching gas.

Moreover, the dry-etching method for preparing a photomask of achromium-containing film is disclosed in, for instance, Japanese PatentNo. 2,765,065, the disclosure of which is hereby incorporated byreference. This patent discloses, in Examples, that when thechromium-containing film is dry-etched by this dry-etching method whileusing a resist film of a positive electron beam resist EBR-9 (which isavailable from Toray Industries, Inc.) as a mask and a mixed gascomprising 160 SCCM of chlorine gas, 40 SCCM of oxygen gas and 160 SCCMof wet air as a dry-etching gas, there is not observed any change in theetching rate of the electron beam resist film, while the etching rate ofthe chromium-containing film increases and the selective (or etching)ratio against the resist film is improved. As a result, thechromium-containing film can sufficiently be patterned by thisdry-etching technique. In this connection, the wet air (160 SCCM) in themixed etching gas comprises about 128 SCCM of nitrogen gas and about 32SCCM of oxygen gas corresponding to the component ratio of nitrogen tooxygen in the air which is equal to 4:1.

In addition, the semiconductor circuit has recently become more and morefiner and the size of the semiconductor circuit is increasingly reducedfrom 0.2 μm to 0.15 μm. For instance, in case of a semiconductor circuitfabricated using a conventional photomask, the dimensional errorobserved for the memory circuit portion is large as compared with thatobserved for the peripheral circuit portion in the memory circuit whichcomprises the memory circuit portion and the peripheral circuit portion,while such an error is also large even in the logic circuit and thusthese errors may adversely affect the characteristic properties of theresulting circuit. For this reason, there has been desired for thedevelopment of a half-tone phase-shift photomask which permits thefabrication of a circuit whose dimensional difference between circuitswithin a semiconductor chip is as low as possible.

If a chromium or chromium oxide film as a light-shielding film or ahalf-tone phase-shift film is subjected to dry-etching using achlorine-containing gas and if a pattern is formed on a plane at analmost uniform density, the film can be chromium-etched at anapproximately uniform rate throughout the whole surface and accordingly,the dimensional control within a plane can be achieved to such an extentthat the in-plane uniformity 3 σ (3× the variance of (measured linewidth−averaged line width)) ranges from 20 to 60 nm for the line widthranging from 1 to 2 μm.

However, dense patterns (patterns whose area occupied by a resist issmall) and coarse patterns (patterns whose area occupied by a resist islarge) often coexist in the plane of a practical photomask and if thedry-etching technique is used for forming such a photomask, the etchingrate of a chromium film is high at the densely patterned portion and lowat the coarsely patterned portion. As a result, the dimensionaldifference within a plane reaches up to about 100 nm for a designed linewidth ranging from 1 to 2 μm. A photomask having such a largedimensional difference within the plane cannot be used for thefabrication of, for instance, circuits having a higher integrationdensity such as memory circuits, logic circuits and LSI circuits.

The Japanese Patent No. 2,765,065 described above does not relate to thesolution of the foregoing problems, but relates to the improvement ofthe selective ratio of a chromium film to a resist film.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is generally to solvethe foregoing problems associated with the conventional technique or animproved dry-etching technique as a means for forming a fine pattern,which permits the reduction of the dimensional difference due to thecoexistence of coarse and dense patterns within a plane, for instance, adry-etching technique for manufacturing a chromium-containing half-tonephase-shift photomask. More specifically, it is an object of the presentinvention to provide a dry-etching method and a dry-etching apparatus,which permit the production of a high precision photomask by reducingthe dimensional difference due to the coexistence of coarse and densepatterns within a plane.

Another object of the present invention is to provide a method forpreparing a photomask using the foregoing dry-etching method and toprovide a photomask thus prepared.

A further object of the present invention is to provide a method forfabricating a semiconductor device using the photomask and asemiconductor circuit fabricated by the method.

The inventors of the present invention have conducted various studies toachieve the foregoing objects, have found that even in the production ofa chromium-containing half-tone phase-shift photomask in which densepatterns and coarse patterns coexist in the plane thereof, the use of amixed etching gas comprising an oxygen-containing halogen gas such as anoxygen-containing chlorine gas (e.g., Cl₂+O₂), to which at least ahydrogen-containing gas (e.g., H₂, hydrogen chloride (HCl) gas) isadded, in the etching of the chromium-containing half-tone phase-shiftfilm permits the achievement of in-plane dimensional control almostidentical to that achieved for a mask in which patterns are formed inthe plane at an almost uniform density, i.e., such in-plane dimensionalcontrol that the dimensional difference is not more than a half of thatconventionally attained, for instance, 10 to 20 nm (0.010 to 0.020 μm)and thus have completed the present invention on the basis of such afinding for the designed line width ranging from 1 to 2 μm.

According to a first aspect of the present invention, there is provideda dry-etching method characterized by using, as an etching gas, a mixedgas including (a) a reactive ion etching gas, which contains anoxygen-containing gas and a halogen-containing gas, and (b) a reducinggas added thereto, in a process for dry-etching a metal thin film as achromium-containing half-tone phase-shift film.

According to a second aspect of the present invention, there is provideda method for preparing a chromium-containing half-tone phase-shiftphotomask by performing a series of pattern-forming steps such as a stepfor forming a resist layer on a photomask blank, a step for exposing andpatterning the resist layer, a developing step, a step for etching thephotomask blank and a step for removing the resist layer and which ischaracterized in that patterns to be transferred onto a wafer are formedon the photomask blank for the chromium-containing half-tone phase-shiftphotomask according to the dry-etching method described above to thusgive a photomask.

According to a third aspect of the present invention, there is provideda chromium-containing half-tone phase-shift photomask which is preparedthrough a series of pattern-forming steps such as a step for forming aresist layer on a photomask blank, a step for exposing and patterningthe resist layer, a developing step, a step for etching the photomaskblank and a step for removing the resist layer and which ischaracterized in that patterns to be transferred onto a wafer are formedon the photomask blank for the chromium-containing half-tone phase-shiftphotomask according to the dry-etching method described above to thusgive a photomask.

According to a fourth aspect of the present invention, there is provideda method for manufacturing a semiconductor circuit which comprises thesteps of transferring the patterns formed on the chromium-containinghalf-tone phase-shift photomask according to the third aspect of theinvention on a wafer on which a light-sensitive material is coated,developing the light-sensitive material to form resist patterns on thewafer, or to manufacture a semiconductor circuit which comprisescoexisting coarse and dense patterns corresponding to the resistpatterns.

According to a fifth aspect of the present invention, there is provideda semiconductor circuit which has a circuit comprising coexisting coarseand dense patterns corresponding to the resist patterns formed bytransferring the resist patterns formed on the chromium-containinghalf-tone phase-shift photomask according to the third aspect of theinvention on a wafer on which a light-sensitive material is coated andthen developing the light-sensitive material.

According to a sixth aspect of the present invention, there is provideda dry-etching apparatus used in dry-etching a metal thin film as achromium-containing half-tone phase-shift film, which is provided with asequencer for establishing dry-etching conditions, wherein the metalthin film is a chromium-containing half-tone phase-shift film consistingof a chromium film, a chromium oxide film, a chromium nitride film,chromium oxynitride film, chromium fluoride film or a laminated filmthereof; wherein if an etching gas used consists of chlorine, oxygen andhydrogen gases, the relative flow rates of these gases as expressed interms of % by volume range from 66 to 46, 17 to 11 and 18 to 41% byvolume, respectively, or if an etching gas used consists of chlorine,oxygen and hydrogen chloride gases, the relative flow rates of thesegases as expressed in terms of % by volume range from 58 to 44, 15 to 11and 28 to 45% by volume, respectively; and wherein the apparatus isdesigned in such a manner that when inputting the parameters relating tothe foregoing dry-etching conditions, directly or through a memorydevice of a computer, to the sequencer and then starting the dry-etchingprocess, the dry-etching is automatically carried out under theforegoing dry-etching conditions.

According to a seventh aspect of the present invention, there isprovided a dry-etching apparatus which comprises an etching chamber, atransport chamber, a substrate cassette bed and a sequencer forestablishing dry-etching conditions, wherein four electromagnets eachcomprising a square-shaped ring-like coil are provided on the outer sideof the etching chamber, two each of these electromagnets being oppositeto one another and making a pair, these electromagnets being so designedthat when applying a low frequency current which is 90 deg. out of phasethereto, the combined magnetic field established by these two pairedelectromagnets can rotate in a plane parallel to a substrate at afrequency identical to that of the low frequency current, an RFelectrode and an opposite electrode are disposed in the etching chamber,a transport robot for transporting the substrate is provided in thetransport chamber, the transport robot being a two-joint robot havingtwo knots, the tip of a transport arm thereof being able to undergoadvancing, reciprocating and rotating motions due to the composition ofrotational motions of a motor axis and these two knots within eachhorizontal plane, the robot thus transporting the substrate, wherein ametal thin film to be dry-etched is a chromium-containing half-tonephase-shift film consisting of a chromium film, a chromium oxide film, achromium nitride film, chromium oxynitride film, chromium fluoride filmor a laminated film thereof, wherein if an etching gas used consists ofchlorine, oxygen and hydrogen gases, the relative flow rates of thesegases as expressed in terms of % by volume range from 66 to 46, 17 to 11and 18 to 41% by volume, respectively, or if an etching gas usedconsists of chlorine, oxygen and hydrogen chloride gases, the relativeflow rates of these gases as expressed in terms of % by volume rangefrom 58 to 44, 15 to 11 and 28 to 45% by volume, respectively, andwherein the apparatus is designed in such a manner that when inputtingthe parameters relating to the foregoing dry-etching conditions,directly or through a memory device of a computer, to the sequencer andthen starting the dry-etching process, the dry-etching is automaticallycarried out under the foregoing dry-etching conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other objects, features and advantages of thepresent invention will be become more apparent from the followingdescription taken with reference to the accompanying drawings, wherein

FIG. 1 is a cross sectional view showing the structure of a photomaskblank;

FIG. 2 is a mask process flow diagram for explaining the process forpreparing a photomask;

FIG. 3 is a partially cutaway plan view showing a dry-etching apparatusused for carrying out the present invention;

FIG. 4 is a cross sectional view of the dry-etching apparatus shown inFIG. 3 taken along the line A-A;

FIG. 5(A) is a schematic plan view showing the arrangement of testpatterns used in Example 1 and FIG. 5(B) shows the pattern arrangementas shown in FIG. 5(A) and is a schematic plan view showing the positions(or lines A-A′ and B-B′) along which the cross sectional views shown inFIGS. 6(A) and 6(B) are taken;

FIG. 6(A) is a flow diagram for explaining the preparation of measuringpatterns (or test patterns) at a densely patterned portion along theline (A-A′) in FIG. 5(B), and FIG. 6(B) is a flow diagram for explainingthe preparation of measuring patterns at a coarsely patterned portionalong the line (B-B′) in FIG. 5(B);

FIG. 7 is a graph showing the influence of the addition of hydrogen gasor hydrogen chloride gas to the etching gas, on the change of thedimensional difference due to the coexistence of coarse and densepatterns observed when the test pattern shown in FIG. 5 is formed byetching;

FIG. 8 is a schematic diagram showing the structure of a memory circuitof the present invention, which comprises a memory circuit portion and aperipheral circuit portion; and

FIG. 9 is a schematic diagram showing the structure of a system LSIcircuit of the present invention which comprises combined memory circuitand logic circuit portions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The dry-etching method according to the present invention is carried outusing, as a reactive etching gas, a mixed gas comprising (a) a reactiveetching gas, which consists of an oxygen-containing gas and ahalogen-containing gas, and (b) a reducing gas added thereto in theprocess for dry-etching a metal thin film. This metal thin film may be athin film such as a chromium film, a chromium oxide film, a chromiumnitride film, a chromium oxynitride film, a chromium fluoride film or alaminated film thereof.

In addition, the reducing gas used herein may be a gas containing atleast hydrogen, i.e., hydrogen gas; a hydrocarbon gas selected from thegroup consisting of C_(n)H_(2n+2)(n=1 to 8), C_(n)H_(2n)(n=2 to 10),C_(n)H_(2n−2)(n=2 to 8); an alcoholic gas selected from the groupconsisting of CH₃OH, C₂H₅OH, CH₃CH₂CH₂OH, (CH₃)₂CHOH, (CH₃)₃COH,CH₂═CHCH₂OH; a hydrogen halide gas selected from the group consisting ofHF, HCl, HBr and HI; ammonia gas; or water.

If the metal thin film or the chromium-containing half-tone phase-shiftfilm is a chromium film, a chromium oxide film, a chromium nitride film,chromium oxynitride film, chromium fluoride film or a laminated filmthereof and if the etching gas or the mixed gas used consists ofchlorine, oxygen and hydrogen gases, the flow rates of these gases asexpressed in terms of % by volume preferably range from 66 to 46, 17 to11 and 18 to 41 % by volume, respectively, while if the metal thin filmis the foregoing chromium-containing half-tone phase-shift film and ifthe mixed gas used consists of chlorine, oxygen and hydrogen chloridegases, the flow rates of these gases as expressed in terms of % byvolume preferably range from 58 to 44, 15 to 11 and 28 to 45% by volume,respectively. This is because if the flow rates each is beyond theforegoing range, it is difficult to obtain a highly precision half-tonephase-shift mask having a reduced dimensional difference due to thecoexistence of coarse and dense patterns in a plane.

Moreover, the oxygen-containing gas used in the invention may be agaseous oxygen-containing compound which can be an oxygen source, suchas O₂, CO, CO₂, NO and N₂O and the halogen-containing gas usable hereinmay be a gaseous halogen (such as chlorine, fluorine)-containingcompound such as chlorine gas, CCl₄, CF₂Cl₂, CFCl₃ or CF₃Cl, which is acommonly used reactive ion etching gas.

The dry-etching apparatus used for practicing the dry-etching methodaccording to the present invention is not restricted to any particularone and may be, for instance, an apparatus which makes use of a varietyof systems such as barrel type, RIE, MERIE, ICP, NLD and ECR. Preferredare those depicted in FIGS. 3 and 4, which are equipped with a sequencerfor establishing dry-etching conditions, wherein the metal thin film isa chromium-containing half-tone phase-shift film consisting of achromium film, a chromium oxide film, a chromium nitride film, chromiumoxynitride film, chromium fluoride film or a laminated film thereof;wherein if the mixed gas used as etching gas consists of chlorine,oxygen and hydrogen gases, the flow rates of these gases as expressed interms of % by volume range from 66 to 46, 17 to 11 and 18 to 41% byvolume, respectively, or if the etching gas used consists of chlorine,oxygen and hydrogen chloride gases, the flow rates of these gases asexpressed in terms of % by volume range from 58 to 44, 15 to 11 and 28to 45% by volume, respectively; and wherein the apparatus is designed insuch a manner that when inputting the parameters relating to theforegoing dry-etching conditions, directly or through a memory device ofa computer, to the sequencer and then starting the dry-etching process,the dry-etching is automatically carried out under the foregoingdry-etching conditions.

The dry-etching apparatus equipped with the foregoing sequenceraccording to the present invention comprises an etching chamber, atransport chamber and a substrate cassette bed, wherein fourelectromagnets each comprising a square-shaped ring-like coil areprovided on the outer side of the etching chamber, two each of theseelectromagnets being opposite to one another and making a pair, theseelectromagnets being so designed that when applying a low frequencycurrent which is 90 deg. out of phase thereto, the combined magneticfield established by these two paired electromagnets can rotate in aplane parallel to the substrate at a frequency identical to that of thelow frequency current, an RF electrode and an opposite electrode beingdisposed in the etching chamber, a transport robot for transporting thesubstrate being provided in the transport chamber, the transport robotbeing a two-joint robot having two knots, the tip of the transport armthereof being able to undergo advancing, reciprocating and rotatingmotions due to the composition of rotational motions of a motor axis andthese two knots within each horizontal plane, and the robot thustransporting the substrate.

In Examples given later, all of the pressure, RF electric power,magnetic field, distance between electrodes, kinds of etching gases andthe flow rate ratio: Cl₂/O₂ in the etching gas mixture are fixed topredetermined values respectively, but they are not restricted to thesespecific values and the dry-etching operations may be performed underthe following conditions, if an MERIE apparatus is, for instance, used:Pressure: 1.3 to 66.7 Pa (10 to 500 mTorr) RF Electric Power:  10 to 300W (RF Electric Power density: 0.10 to 0.4 W/cm²) Flow Rate Ratio,O₂/(Cl₂ + O₂):  10 to 25% Cl₂/O₂:  20 to 160/5 to 100 SCCM MagneticField:   0 to 150 Gs Interelectrode Distance:  40 to 120 mm

According to the present invention, a chromium-containing half-tonephase-shift mask can be prepared by a series of well-knownpattern-forming steps such as a step for forming a resist layer on aphotomask blank, a step for exposing and patterning the resist layer, adeveloping step, a step for etching the photomask blank and a step forremoving the resist layer, wherein patterns to be transferred onto awafer are formed on the photomask blank using the dry-etching methoddescribed above as the dry-etching process. In addition, thechromium-containing half-tone phase-shift mask of the present inventionis characterized in that patterns to be transferred onto a wafer areformed on a photomask blank using the dry-etching method described aboveas the dry-etching process among a series of the foregoing well-knownpattern-forming steps.

The present invention further permits the manufacture of a semiconductorcircuit by transferring the resist patterns formed on thechromium-containing half-tone phase-shift photomask produced by theforegoing method on a wafer on which a light-sensitive material iscoated, developing the light-sensitive material to form the resistpatterns on the wafer, and then subjecting the wafer to etching such asdry-etching or ion-implantation on the basis of the resist patterns thusformed on the wafer to thereby form a circuit having patternscorresponding to the resist patterns. Examples of semiconductor circuitsthus obtained include a memory circuit in which patterns are regularlyarranged, a logic circuit comprising randomly arranged patterns, and asystem LSI circuit comprising combined memory and logic circuits.

The following are characteristic properties of the semiconductor circuitobtained using the chromium-containing half-tone phase-shift mask of thepresent invention. For instance, a memory circuit comprises a memorycircuit portion on which patterns are regularly arranged and aperipheral circuit portion on which patterns are irregularly arranged toensure the connection to the exterior and therefore, the areas occupiedby the patterns in these circuit portions are different from oneanother. More specifically, in a gate-forming process in manufacturing atransistor which has an important influence upon the characteristics ofthe resulting circuit, the rate of area to be removed for patterning inthe peripheral circuit portion is high as compared with that observedfor the memory cell portion. The semiconductor circuit has recentlybecome more and more finer and the size of the semiconductor circuit isincreasingly reduced from 0.2 μm to 0.15 μm. In case of thesemiconductor circuit fabricated using the chromium-containing half-tonephase-shift mask according to the present invention, the dimensionaldifference observed between the memory cell and peripheral circuitportions is very small, the variation in the dimension is also small andtherefore, the characteristics of the circuit are not adversely affectedat all. For this reason, the present invention permits the manufactureof an excellent semiconductor circuit whose memory cell and peripheralcircuit portions have almost the same characteristic properties.

The same effect is also observed for the logic circuit in which patternsare randomly arranged and the distribution of the area to be removed forpatterning is also random and thus the invention permits the manufactureof a quite excellent semiconductor circuit having a very low dimensionaldifference within the chip. The present invention can further be appliedto the production of a system LSI circuit comprising a combination of amemory circuit portion and a logic circuit portion. In this case, thememory circuit portion and the logic circuit portion differ from eachother in the packing densities of devices and the densities of wiring,but the dimensional difference between the memory and logic circuitportions is very small and therefore, does not adversely affect thecharacteristic properties of the resulting circuit. As a result, a goodsemiconductor circuit can thus be produced, which does not have anydifference between the memory and logic circuit portions in theircharacteristics.

The present invention will hereinafter be described in more detail withreference to the following Examples and attached figures, but theseExamples are given only for the purpose of illustration and the presentinvention is not restricted to these specific Examples at all.

The dry-etching apparatus (MERIE apparatus) used in the followingExamples is shown in FIGS. 3 and 4. This dry-etching apparatus 1 is sodesigned that an etching chamber 2, a transport chamber 3 and asubstrate cassette bed 4 are accommodated in a panel 12 whichconstitutes the outer periphery of the dry-etching apparatus and thatthe etching chamber 2 is accommodated in electromagnets 5, 6, 7, 8disposed on the outer periphery of the chamber 2. Each electromagnetcomprises a square-shaped ring-like coil, the electromagnets 5 and 6 andthe electromagnets 7 and 8 make pairs respectively and a low frequencycurrent whose phase is shifted, for instance, 90 deg. is passed throughthese electromagnets. These electromagnets are so designed that thecoils of the paired electromagnets are wound in the same direction andthe combined magnetic field established by these two pairedelectromagnets 5 and 6, and 7 and 8 can rotate in a plane parallel tothe substrate at a frequency identical to that of the low frequencycurrent as shown by the dotted arrows in FIGS. 3 and 4. Disposed withinthe etching chamber 2 are a plate-like RF electrode 10 which isconnected to an RF power supply 9 through a condenser 13 and aplate-like opposite electrode 14 and a substrate 11 may be placed on theRF electrode 10 through a substrate delivery port 15 formed on the sideof the etching chamber 2.

The opposite electrode 14 and the etching chamber 2 are maintained atthe ground voltage. To supply a reactive gas for etching to the etchingchamber 2, a gas supply system 16 is disposed at a reactive gas supplyport 30, which is provided with a gas bomb and a mass flow controllerand an exhaust system 17 is connected to a vacuum exhaust port 18 of theetching chamber 2, which is equipped with a vacuum pump for controllingthe gas pressure in the etching chamber 2. The reactive gas herein usedis one comprising an oxygen-containing mixed gas and a reducing gas atleast containing hydrogen, as has already been discussed above.

A plurality of substrates 11 are accommodated in a cassette case 19,then the cassette case is put on the substrate cassette bed 4, eachsubstrate delivered from the cassette case 19 is brought into thetransport chamber 3 through a partition valve 20 by the action of atransport robot 21 and then placed on the RF electrode 10 in the etchingchamber 2 through a vacuum valve 22 and the substrate delivery port 15.The transport robot 21 is a known two-joint robot having two knots 26and 28 and is so designed that the tip of a transport arm 29 thereof canundergo reciprocating and rotating motions due to the composition ofrotational motions of a motor axis 24 and these two knots 26 and 28. Themotions of a first arm 25 and a second arm 27 are restricted to onlythose within the horizontal planes. The movement of the tip of thetransport arm 29 between the cassette case 19 and the transport chamber3 through the partition valve 20 and that of the tip of the transportarm 29 between the RF electrode 10 in the etching chamber 2 and thetransport chamber 3 through the substrate delivery port 15 are advancingand reciprocating motions due to the composition of rotational motionsof each arm 25, 27, 29 of the robot at the motor axis 24 and theforegoing two knots 26 and 28. The transport of the substrate betweenthe vacuum valve 22 at the upstream side of the substrate delivery port15 and the partition valve 20 on the side of the cassette case 19 isperformed by the half turn motion, within a horizontal plane, of thetransport arm 29 of the robot, wherein the motor axis 24 serves as arotating center. If a motor 23 disposed on the exterior of the transportchamber 3 is rotated, the plate-like transport arm 29 carrying asubstrate 11 undergoes reciprocating and rotational motions to thustransport the substrate between the cassette case 19 and the RFelectrode 10.

When etching a pattern-forming material of the substrate 11 on the RFelectrode 10, the etching chamber 2 is evacuated by operating theexhaust system 17, followed by introduction of a reactive gas into thechamber 2 through the reactive gas supply port 30, excitation of the twopairs of electromagnets 5, 6, 7, 8 and application of an RF electricpower to the RF electrode 10 to thus generate plasma. If the same lowfrequency alternating current is passed through these two pairedelectromagnets 5 and 6, and 7 and 8 in the same direction, whileshifting the phase of the current applied to either of the pairedelectromagnets 90 deg. relative to that of the other current, a rotatingmagnetic field is established in a plane parallel to the substrate 11.The plasma generated between the RF electrode 10 and the oppositeelectrode 14 is concentrated on the surface of the substrate 11 by theaction of the rotating magnetic field and this leads to an increase ofits density. Thus, the reactive gas introduced into the chamber ishighly efficiently dissociated and the substrate 11 is subjected toreactive ion-etching under such a condition that only a slight DC biasvoltage is generated on the substrate.

For instance, it is assumed that the dry-etching is carried out using,as the substrate 11, a photomask substrate which comprises a transparentsubstrate of synthetic quartz or the like, a thin layer of apattern-forming material such as Cr, Cr provided with an antireflectionfilm or SiO₂ which is applied onto the transparent substrate and apatterned photoresist layer (for instance, a resist for EB exposure(e.g. ZEP-810S (trade name) available from Nippon Zeon CO., Ltd.))provided on the pattern-forming material. In this case, if the substrate11 is dry-etched with the introduced reactive gas under the sameconditions used for the dry-etching of the IC substrate, this results ininsufficient selection ratio of the material to be etched to the resistand insufficient in-plane dimensional uniformity of the photomask sincethe photomask substrate 11 is not made of silicon, but is made fromsynthetic quartz unlike the IC substrate and the pattern-formingmaterial applied thereon does not comprise poly Si or an oxidelayer+poly Si, but comprises Cr, Cr provided with an antireflection filmor SiO₂, unlike the IC substrate. However, the selection ratio and thein-plane dimensional uniformity can be improved and a highly precisionphotomask can be produced if establishing the following dry-etchingconditions: a magnetic field intensity ranging from 50 to 150 Gs; apressure of the reactive gas in the etching chamber 2 ranging from 0.03to 0.3 Torr (4 to 40 Pa); and an RF electric power density on the RFelectrode 10 ranging from 0.20 to 0.32 W/cm².

The following are specific Examples of the present invention.

Example 1

A dry-etching method carried out according to the present invention willbe described in this Example. Test patterns used in this Example areshown in FIGS. 5(A) and 5(B) and the flow diagrams for illustrating thepreparation of test samples are shown in FIGS. 6(A) and 6(B). FIG. 6(A)is a flow diagram, as expressed in terms of schematically crosssectional diagrams, for explaining the preparation of measuring patternsat a densely patterned portion along the line (A-A′) in FIG. 5(B), andFIG. 6(B) is a flow diagram, as expressed in terms of schematicallycross sectional diagrams, for explaining the preparation of measuringpatterns at a coarsely patterned portion along the line (B-B′) in FIG.5(B). As will be clear from the flow diagrams shown in FIGS. 6(A) and(B), each test sample was prepared through processes for (a) EBpatterning, (b) developing, (c) etching and (d) removing the resist. InFIGS. 6(A) and (B), d represents a substrate, e represents, in thiscase, a chromium oxide film and f represents a resist layer. The detailsof the etching processes shown in FIGS. 6(A) and (B) are as follows:

The conditions for operating the MERIE apparatus as shown in FIGS. 3 and4, which permit the dry-etching of a chromium film or a chromiumoxynitride film using a conventional chlorine-containing gas havealready been described above, but in this Example, the test patterns asshown in FIGS. 5(A) and (B) were formed by dry-etching a photomask blankfor chromium oxide half-tone phase-shift masks under the conditionsdisclosed in Table 1, using an etching gas comprising the foregoing gassystem (Cl₂/O₂=80/20 SCCM) to which hydrogen gas was added in an amountspecified in the following Table 1 and using the MERIE apparatus asshown in FIGS. 3 and 4, on the basis of the following working conditionsof the apparatus among others: Pressure: 6.8 Pa (50 mTorr) RF PowerSupply: 80 W Gas: Cl₂/O₂ = 80/20 SCCM Magnetic Field: 50 to 60 GsInterelectrode Distance: 60 mm

The photomask blank for chromium oxide half-tone phase-shift mask usedin this Example was one prepared by applying a single chromium oxidelayer in a thickness of about 900 Å as a pattern-forming material ontothe surface of a square-shaped synthetic quartz substrate having a sizeof 152.4×152.4 mm and a thickness of 6.35 mm and then applying a resistlayer (ZEP-810S) for EB exposure onto the layer of the pattern-formingmaterial. Moreover, the test patterns as shown in FIGS. 5(A) and (B)comprised, after the EB exposure and the development, adimension-evaluation pattern (about 6.5 mm×35 mm) which was arranged atthe central portion of the left half (dense portion) of the mask andwhich included a plurality of L/S (Line and Space), isolated L andisolated S patterns therein; a completely removed pattern (the patternof the exposed chromium oxide having a size of 46 mm×54 mm) surroundingthese patterns; and a dimension-evaluation pattern which was arranged atthe central portion of the right half (coarse portion) of the mask.After the EB exposure and the development, the dry-etching was carriedout using hydrogen gas as the added gas. The pattern-forming conditionsare summarized in Table 1.

Shown in FIG. 7 are the results thus observed when adding 0 to 72 SCCMof hydrogen gas to the etching gas and forming test patterns (FIGS. 5(A)and (B)) which comprised dense and coarse patterns arranged in a plane,or the variation of the dimensional difference between the coarse anddense portions (i.e., the difference in the dimension between the coarseand dense portions) as a function of the added amount of the hydrogengas. In addition, shown in the following Table 2 are the changes in thedimension of the coarse and dense portions as a function of the amountof the hydrogen gas which is added to the etching gas when the testpatterns are formed through dry-etching. Moreover, shown in thefollowing Table 3 are the dimensional variation in the coarse and denseportions of the resist pattern obtained after the development as afunction of the added amount of the hydrogen gas.

The following were found, as a result of the dry-etching performed underthe foregoing conditions:

As will be seen from the results shown in Table 2 and FIG. 7, there isobserved a dimensional difference of about 0.053 μm between the coarseand dense portions when any hydrogen gas was not added to the etchinggas, while the dimensional difference therebetween is reduced as theadded amount of the hydrogen gas increases. More specifically, when anincreasing amounts of hydrogen gas ranging from 21.6 to 72 SCCM wereadded to, in order, Sample Nos. 1-2, 1-3, 1-4 and 1-5, the dimensionaldifference between the coarse and dense portions of the pattern wasfound to be about 0.011 to 0.002 μm which corresponded to not more thanabout ⅕ to {fraction (1/26)} times that (0.053 μm) observed when anyhydrogen gas was not added. This clearly indicates that the problem ofthe dimensional difference between the coarse and dense portions isconsiderably eliminated. As shown in Table 1, the gas flow rates of Cl₂,O₂ and H₂ and the relative flow rates (as expressed in terms of % byvolume) used in the dry-etching of the samples 1-2, 1-3, 1-4 and 1-5 are80, 20 and 21.6 to 72 SCCM and 65.79 to 46.51, 16.45 to 11.63 and 17.76to 41.86% by volume, respectively.

As has been discussed above, it has been proved that the conditions fordry-etching the samples 1-2, 1-3, 1-4 and 1-5 are optimum ones andaccordingly, the apparatus was so designed that the parameters relatingto the foregoing dry-etching conditions listed in Table 1 were inputted,directly or through a memory device of a computer, to the sequencer andthen the dry-etching process was started to automatically carry out thedry-etching under the foregoing optimum dry-etching conditions.

Example 2

A dry-etching method carried out according to the present invention willbe described in this Example. The photomask blank for preparing achromium oxide half-tone phase-shift mask and the test patterns used inthis Example were the same as those used in Example 1. The scheme forpreparing the test samples were the same as that used in Example 1except that hydrogen chloride (IHCI) gas, as the gas to be added to theetching gas, was substituted for the hydrogen gas used in Example 1 inthe etching step, that ZEP7000 (the trade name of a product availablefrom Nippon Zeon CO., Ltd.) was substituted for the ZEP-810S as theresist for the EB exposure and that the resist film formed had athickness of 4000 Å. The pattern-forming conditions are listed in thefollowing Table 1 together with those used in Example 1.

HCl gas was added to the etching gas in an amount ranging from 0 to 140SCCM and test patterns (FIGS. 5(A) and (B)) having coarse and denseportions in a plane were formed by etching. Regarding the resultsobserved after carrying out the etching under the conditions specifiedin Table 1, the change in the averaged dimensional difference betweenthe coarse and dense portions (averaged value of the difference betweenthe dimensions of the coarse and dense portions) is shown in FIG. 7 as afunction of the added amount of the HCl gas while comparing theseresults with those obtained in Example 1 in which hydrogen gas was usedas the added gas component. In addition, listed in Table 2 are thedimensional variation in the coarse and dense portions as a function ofthe added amount of HCl gas observed when the test patterns were formedthrough dry-etching, together with those observed in Example 1 whereinhydrogen gas was used as the added gas. In addition, the dimensions ofthe resist patterns after the development are the same as those listedin Table 3 which are observed when using hydrogen gas as the added gas.

As will be seen from the data shown in Table 2 and FIG. 7, there isobserved a dimensional difference of about 0.052 μm between the coarseand dense portions when any HCl gas was not added to the etching gas,while the dimensional difference therebetween was reduced as the addedamount of the HCl gas increased. More specifically, the results ofSample Nos. 2-2, 2-3 and 2-4 indicate that when HCl gas was added in anamount ranging from 40 to 80 SCCM, the dimensional difference betweenthe coarse and dense portions of the patterns was in the range of fromabout 0.010 to 0.020 μm which corresponded to about 1/5.2 to 1/2.6 timesthat observed when any HCl gas was not added. This clearly indicatesthat the problem of the dimensional difference between the coarse anddense portions is considerably eliminated. The result of dimensionaldifference may further be improved by optimizing the relative flow ratesof gas components in the etching gas. As shown in Table 1, the gas flowrates of Cl₂, O₂ and HCl and the relative flow rates (expressed in termsof % by volume) used in the dry-etching of the samples 2-2, 2-3 and 2-4are 80, 20 and 40 to 80 SCCM; and 57.14 to 44.44, 14.28 to 11.11 and28.57 to 44.44% by volume, respectively.

Moreover, the etching operation may be safer by the use of HCl gas inplace of hydrogen gas as the added gas.

As has been described above in detail, it has been proved that theconditions for dry-etching the samples 2-2, 2-3 and 2-4 are optimum onesand accordingly, the apparatus was so designed that the parametersrelating to the foregoing dry-etching conditions listed in Table 1 wereinputted, directly or through a memory device of a computer, to thesequencer and then the dry-etching process was started to automaticallycarry out the dry-etching under the foregoing optimum dry-etchingconditions. TABLE 1 Pattern-Forming Conditions (Cr-Containing Half-Tone)Resist Layer Dry-Etching Conditions Thickness Inter- Just Total Added ofAdded RF Power Magnetic electrode Etching Etching Etching Resist Cl₂ O₂Gas Supply Pressure Field Distance Time Time Ex. No. Gas Sample No.Resist A SCCM SCCM SCCM W Pa Gauss mm sec sec 1 H₂ 1-1 ZEP810S 5000 (80)(20) (0) 80 6.8 50-60 60 161 483 80 20  0 1-2 ″ ″ (65.79) (16.45)(17.76) ″ ″ ″ ″ 106 318 80 20  21.6 1-3 ″ ″ (60.38) (15.09) (24.53) ″ ″″ ″ 120 360 80 20  32.5 1-4 ″ ″ (55.83) (13.96) (30.22) ″ ″ ″ ″ 112 33680 20  43.3 1-5 ″ ″ (46.51) (11.63) (41.86) ″ ″ ″ ″ 127 381 80 20  72 2HCl 2-1 ZEP7000 4000 (80) (20) (0) 80 6.8 50-60 60 161 483 80 20  0 2-2″ ″ (57.14) (14.28) (28.57) ″ ″ ″ ″ 85 255 80 20  40 2-3 ″ ″ (50.00)(12.50) (37.50) ″ ″ ″ ″ 96 288 80 20  60 2-4 ″ ″ (44.44) (11.11) (44.44)″ ″ ″ ″ 90 270 80 20  80 2-5 ″ ″ (33.33) (8.33) (58.33) ″ ″ ″ ″ 102 30680 20 140(Note)The numerical value in the parenthesis corresponds to the flow rateexpressed in terms of “by volume”.

TABLE 2 Dimensions of Patterns Observed After Dry-Etching (Cr-ContainingHalf-Tone) Flow Dense Portion Coarse Portion Difference Between Coarseand Dense Portions Rate of (Large Removed Area) (Small Removed Area)(Coarse-Dense) Added Added [μm] [μm] [μm] Ex. Etching Sample Gas L/S L/SISO ISO L/S L/S ISO ISO L/S L/S ISO ISO Average No. Gas No. SCCM LineSpace Line Space Line Space Line Space Line Space Line Space (AbsoluteValue) 1 H₂ 1-1 0 1.999 2.007 1.993 2.012 2.055 1.960 2.052 1.962 0.056−0.047 0.059 −0.050 0.053 1-2 21.6 1.997 2.010 1.987 2.010 1.986 2.0191.980 2.026 −0.011 0.009 −0.007 0.016 0.011 1-3 32.5 1.967 2.041 1.9492.047 1.963 2.044 1.950 2.047 −0.004 0.003 0.001 0.000 0.002 1-4 43.31.462 2.039 1.956 2.038 1.967 2.034 1.960 2.044 0.005 −0.005 0.004 0.0060.005 1-5 72 1.941 2.059 1.931 2.066 1.950 2.052 1.949 2.056 0.009−0.007 0.018 −0.010 0.011 2 HCl 2-1 0 1.966 2.036 1.949 2.047 2.0171.993 2.016 2.000 0.051 −0.043 0.067 −0.047 0.052 2-2 40 2.017 1.9862.010 1.975 2.010 2.002 2.005 2.011 −0.007 0.016 −0.005 0.036 0.016 2-360 1.893 2.110 1.874 2.101 1.908 2.098 1.887 2.102 0.015 −0.012 0.0130.001 0.010 2-4 80 1.895 2.105 1.878 2.109 1.916 2.079 1.900 2.098 0.021−0.026 0.022 −0.011 0.020 2-5 140 1.864 2.136 1.855 2.142 1.912 2.0851.910 2.087 0.048 −0.051 0.055 −0.055 0.052

TABLE 3 Dimensions of Resist Patterns Observed After Development(Cr-Containing Half-Tone) Difference Between Coarse Flow Dense PortionCoarse Portion and Dense Portions Rate of (Large Removed Area) (SmallRemoved Area) (Coarse-Dense) Added Added [μm] [μm] [μm] Ex. EtchingSample Gas L/S L/S ISO ISO L/S L/S ISO ISO L/S L/S ISO ISO No. Gas No.SCCM Line Space Line Space Line Space Line Space Line Space Line Space 1H₂ 1-1 0 2.119 1.365 2.118 1.355 2.129 1.350 2.126 1.345 0.010 −0.0150.008 −0.010 1-2 21.6 2.135 1.334 2.134 1.344 2.136 1.343 2.135 1.3500.001 0.009 0.001 0.006 1-3 32.5 2.117 1.369 2.114 1.370 2.117 1.3612.116 1.367 0.000 −0.008 0.002 −0.003 1-4 43.3 2.101 1.362 2.106 1.3632.106 1.360 2.108 1.356 0.005 −0.002 0.002 −0.007 1-5 72 2.096 1.3882.089 1.373 2.103 1.383 2.093 1.374 0.007 −0.005 0.004 0.001(Note)L/S: Line and SpaceL: LineS: SpaceISO: Isolated

Example 3

In the light of the results obtained in Examples 1 and 2, it was foundthat the use of hydrogen and hydrogen chloride gases in dry-etching asthe gas component to be added to the etching gas permits the reductionof the dimensional difference due to the coexistence of coarsely anddensely patterned portions in a plane and the formation of highprecision patterned products by etching. Accordingly, there wereprepared several kinds of photomasks according to the procedures used,in Examples 1 and 2, for preparing the test samples carrying testpatterns, or by forming a resist layer on a photomask blank for achromium or chromium oxide half-tone phase-shift photomask, followed bya series of well-known pattern-forming steps such as light-exposure,development, etching and washing. The resulting photomasks each hadpatterns such as hole systems or line-and-space patterns to betransferred to a wafer and comprised, on a plane, coexisting coarse anddense patterns. The resulting chromium oxide half-tone phase-shiftphotomask was found to have a very small dimensional difference betweenthe coarse and dense portions of the patterns. These results clearlyindicate that the use of the foregoing added gas permits the achievementof quite satisfactory results, i.e., substantial reduction of thedimensional difference, like the results observed in Examples 1 and 2.

Example 4

A semiconductor circuit was formed on a wafer by repeating the followingsteps (1) to (4) using the chromium oxide half-tone phase-shiftphotomask prepared in Example 3:

-   (1) A light-sensitive material was applied onto the wafer;-   (2) The patterns on the photomask were scaled down and transferred    to the wafer using a stepper (scale factor: ⅕, ¼ or ½);-   (3) The wafer provided thereon with the exposed light-sensitive    material was developed to form a resist pattern on the wafer;-   (4) The wafer was subjected to dry-etching or ion-implantation    through the resist pattern.

The semiconductor circuits thus produced were a memory circuit (FIG. 8)whose patterns were regularly arranged, a logic circuit whose patternswere randomly arranged and a system LSI circuit (see FIG. 9) comprisingcombined memory and logic circuits.

The memory circuit as shown in FIG. 8 comprises a memory circuit portionwhose patterns are regularly arranged and a peripheral circuit portionin which patterns are irregularly arranged in order to ensure theconnection to the exterior and these circuit portions differ from eachother in the rate of the area occupied by the patterns. In thetransistor gate-forming process which has a serious influence on thecharacteristics of the circuit, the rate of the area removed forpatterning on the peripheral circuit portion increases in proportion toan increase in the size of the memory cell portion, i.e. is higher thanthat of the memory cell portion. In the case of the semiconductorcircuit produced according to the present invention, the dimensionaldifference observed between the memory cell and peripheral circuitportions is very small and on the order of about 0.004 μm and thisindicates that the amount of the dimensional variation is not more than2%. Consequently, the patterns never adversely affect the characteristicproperties of the resulting circuit. Thus, satisfactory semiconductorcircuits could be produced according to the present invention, which didnot show any difference between the memory cell and peripheral circuitportions in characteristic properties.

Moreover, the semiconductor circuit or logic circuit produced in thisExample was found to have excellent quality, since the same effectdescribed above could also be attained by the logic circuit in whichpatterns were randomly distributed and the areas removed for patterningwere also randomly distributed and the dimensional difference within thechip was found to be very small on the order of not more than 0.004 μm.It was also found that the present invention permitted the production ofan excellent semiconductor circuit or a system LSI circuit comprisingcombined memory and logic circuit portions (FIG. 9), which did notdiffer from each other in characteristic properties. This is because thememory and logic circuit portions differ from each other in the densityof patterns, but the dimensional difference between the memory cell andlogic circuit portions is very small on the order of 0.004 μm andtherefore, the patterns never adversely affect the characteristicproperties of the resulting circuit.

As has been discussed above in detail, the method of the presentinvention permits the decrease of the dimensional difference due to thecoexistence of coarsely and densely patterned portions in a plane andthe production of a high precision pattern-etched product by using amixed gas, which comprises (a) a reactive ion etching gas as a mixtureof an oxygen-containing gas and a halogen-containing gas, and (b) areducing gas containing at least hydrogen, in the dry-etching process asa means for forming fine patterns.

Moreover, the method for preparing a chrome type half-tone phase-shiftphotomask according to the present invention permits a photomask whichhas a small dimensional difference due to the coexistence of the coarseand dense patterns in a plane and whose patterns are highly preciselyprocessed. In addition, the photomask has uniform patterns. Further thesemiconductor circuit produced using the photomask of the presentinvention has a very high integration density.

In addition, the use of hydrogen chloride gas instead of hydrogen gas asthe added gas component can ensure safer etching procedures.

1-10. (Cancelled).
 11. A method for preparing a chromium-containinghalf-tone phase-shift photomask by performing a series ofpattern-forming steps such as a step for forming a resist layer on aphotomask blank, a step for exposing and patterning said resist layer, adeveloping step, a step for etching said photomask blank and a step forremoving the resist layer, wherein the method is characterized in thatpatterns to be transferred onto a wafer are formed on said photomaskblank for the chromium-containing half-tone phase-shift photomaskaccording to a dry-etching method comprising dry-etching a metal thinfilm as a chromium-containing half-tone phase-shift film, wherein themethod is characterized by using, as an etching gas, a mixed gasincluding (a) a reactive ion etching gas, which contains anoxygen-containing gas and a halogen-containing gas, and (b) a reducinggas added to the gas component (a), in the process for dry-etching themetal thin film to thus give a photomask.
 12. A method for preparing achromium-containing half-tone phase-shift photomask by performing aseries of pattern-forming steps such as a step for forming a resistlayer on a photomask blank, a step for exposing and patterning saidresist layer, a developing step, a step for etching said photomask blankand a step for removing the resist layer, wherein the method ischaracterized in that patterns to be transferred onto a wafer are formedon said photomask blank for the chromium-containing half-tonephase-shift photomask according to the a dry-etching method comprisingdry-etching a metal thin film as a chromium-containing half-tonephase-shift film, wherein the method is characterized by using, as anetching gas, a mixed gas including (a) a reactive ion etching gas, whichcontains an oxygen-containing gas and a halogen-containing gas, and (b)a reducing gas added to the gas component (a), in the process fordry-etching the metal thin film, wherein said metal thin film is achromium-containing half-tone phase-shift film consisting of a chromiumfilm, a chromium oxide film, a chromium nitride film, chromiumoxynitride film, chromium fluoride film or a laminated film thereof tothus give a photomask.
 13. A chromium-containing half-tone phase-shiftphotomask which is prepared by performing a series of pattern-formingsteps such as a step for forming a resist layer on a photomask blank, astep for exposing and patterning said resist layer, a developing step, astep for etching said photomask blank and a step for removing saidresist layer, wherein the photomask is characterized in that patterns tobe transferred onto a wafer are formed on said photomask blank for thechromium-containing half-tone phase-shift photomask according to adry-etching method comprising dry-etching a metal thin film as achromium-containing half-tone phase-shift film, wherein the method ischaracterized by using, as an etching gas, a mixed gas including (a) areactive ion etching gas, which contains an oxygen-containing gas and ahalogen-containing gas, and (b) a reducing gas added to the gascomponent (a), in the process for dry-etching the metal thin film.
 14. Achromium-containing half-tone phase-shift photomask which is prepared byperforming a series of pattern-forming steps such as a step for forminga resist layer on a photomask blank, a step for exposing and patterningsaid resist layer, a developing step, a step for etching said photomaskblank and a step for removing said resist layer, wherein the photomaskis characterized in that patterns to be transferred onto a wafer areformed on said photomask blank for the chromium-containing half-tonephase-shift photomask according to a dry-etching method comprisingdry-etching a metal thin film as a chromium-containing half-tonephase-shift film, wherein the method is characterized by using, as anetching gas, a mixed gas including (a) a reactive ion etching gas, whichcontains an oxygen-containing gas and a halogen-containing gas, and (b)a reducing gas added to the gas component (a), in the process fordry-etching the metal thin film, wherein said metal thin film is achromium-containing half-tone phase-shift film consisting of a chromiumfilm, a chromium oxide film, a chromium nitride film, chromiumoxynitride film, chromium fluoride film or a laminated film thereof.15-20. (Cancelled).